1. Field of the Invention
The invention relates to a composite sheet steel of maraging steel, especially for the protection of vehicles, especially passenger vehicles and transport vehicles for valuables, against shots and the effect of explosives, wherein the composite sheet steel has a hardened outer layer and a tenacious inner layer.
2. Description of the Related Art
Such a composite sheet steel is known from DE 43 44 879 C2.
A variation and further development of such a sheet steel are disclosed in the German patent application 199 21 961.3. According to this patent application, the steel of the inner layer has a chemical composition in percent by weight of Cxe2x89xa60.01, Sixe2x89xa60.1, Mnxe2x89xa60.1, Pxe2x89xa60.005, Sxe2x89xa60.005, Cuxe2x89xa60.1, Mo 4.80 to 5.20, Ni 17.5 to 18.5, Crxe2x89xa60.1, Ti 0.55 to 0.70, Co 8.0 to 9.0 as well as optionally Al 0.05 to 0.15, and the steel of the outer layer, has a chemical composition, after purification by a zone melting process, in percent by weight of Cxe2x89xa60.01, Si less than 0.1, Mn 0.02 to 0.20, Pxe2x89xa60.005, Sxe2x89xa60.005, Cu 0.01 to 0.20, Mo 4.80 to 5.20, Ni 17.5 to 18.5, Cr 0.01 to 0.20, Ti 1.80 to 1.95, Co 14.0 to 15.5, Al 0.05 to 0.15, with the remainder being Fe and contaminants resulting from the process, respectively. The steel of the outer layer is thus substantially free of oxide inclusions which, according to the assumption of the invention, should be avoided as much as possible because of their notch effect causing cracks, in particular, during loading of the steel when subjected to shots, and this cannot be neglected in this context. A composite sheet steel produced accordingly fulfills especially high requirements.
In general, the brittle outer layer of the composite sheet steels of the aforementioned kind has a tendency to separate from the more tenacious inner layer at a spacing of approximately 5 to 10 cm from the impact location, and complete detachment may occur. Such a detachment can occur despite a proven fixed bonding between the two layers. According to the laws of probability, further impacts will generally occur at different locations, but, when a projectile impacts on the location with detached outer layer, only the inner layer is present and cannot prevent the projectile from penetrating.
It is an object of the present invention to increase the resistance of a composite steel against detachment or separation of the outer layer from the inner layer.
In accordance with the present invention, this is achieved in that an intermediate layer is arranged between the outer layer and the inner layer, wherein the intermediate layer is thinner than the inner and outer layers.
The intermediate layer is not provided for the purpose of serving as a bonding agent for a further strengthening of the bonding between the inner and the outer layers.
The invention is based on the assumption that the transversal vibrations of the sheet steel resulting from the impact of the projectile and the transversal and longitudinal waves spreading from the impact location within the sheet steel can spatially and temporally coincide such that especially high, detachment-causing tensile stress is generated at certain locations, wherein within the two-layer steel the longitudinal waves alone can also form tension peaks caused by the coincidence of continuing partial waves at the interface as a result of the splitting of the longitudinal waves at the interface into a penetrating portion and a reflected portion and the reflection on the two surfaces.
The latter effect is reduced by the invention in that within the thin intermediate layer a system of correspondingly frequently reflected partial waves is generated which prevents the possible coincidence of the waves in the two layers of the two-layer steel by the intermediately positioned disturbance area.
The intermediate layer is comprised, for example, of a nickel foil of the type used as a carbon diffusion barrier in composite sheet steels for container construction etc. Also, a high-tenacity micro-alloyed construction steel can be used.
The thickness of the intermediate layer may be very minimal. Depending on the material, it will be between {fraction (1/100)} and 1 mm.
The bonding of the three layers is performed preferably in one working step by roll-bonding after a corresponding preparation of the surfaces. Depending on the material and the thickness of the intermediate layer, other plating or cladding techniques can be used also.
As a preparation of the roll-bonding process, the two steel plates to be connected should be provided with metallic contact surfaces by means of metal removing machining such as planing or milling. Moreover, before roll-bonding, the plates should be welded together peripherally to be sealed and a high vacuum should be generated in the space between the two plates.
The measures according to the present invention can be combined in an advantageous manner with those of the aforementioned German patent application 199 21 961.3.
Also, these measures according to the present invention can be used in connection with measures disclosed in the German patent application 198 57 156.9 according to which, subsequent to the hardening, an additional surface (face) hardening is performed at a temperature which is identical to the conversion temperature of martensite into austenite.